The invention concerns a hydraulic steering arrangement with a steering handwheel, which is connected with a hydraulic steering unit, and a steering motor, which is connected with a pump via a steering valve.
A steering arrangement of this kind is, for example, known from U.S. Pat. No. 5,234,070. In this connection, the steering valve, in dependence of the direction predetermined by the steering handwheel, releases a connection between the pump and the steering motor, which eventually causes the angle movement of the steered wheels of a vehicle. For this purpose, the connection is made or broken through a slide of a steering valve, which slide can be displaced by means of electromagnetic drives. For an emergency operation during failure of the electromagnetic control, the slide can also be displaced by hydraulic pressures, which are produced by the steering handwheel itself.
From the late published German patent application 198 25 579 is known an additional hydraulic steering arrangement, in which the slide of the steering valve can be displaced either by pressures, which are produced by the steering handwheel or by pressures, which are produced by a control pressure generator. In this connection, the steering hand-wheel, or rather, a steering arrangement connected with the steering handwheel, becomes a steering pressure, which corresponds to the LS pressure of the system. When the wheels are not making a steering movement, the slide of the steering valve is in its neutral position, and the corresponding pressure is relatively low. Further, the LS-line mainly permits the propagation of a pressure. This line is less suited for transporting hydraulic fluids. The supply of the steering unit by means of this xe2x80x9cdynamic flowxe2x80x9d can therefore cause that a so-called xe2x80x9chard pointxe2x80x9d occurs in connection with the steering. This xe2x80x9chard pointxe2x80x9d occurs when in relation to the load to be moved, that is the wheels, the pressure available is not sufficient. This is particularly the case at the beginning of a steering process, and the driver or the operator of a vehicle equipped with such a steering arrangement finds this uncomfortable. Further, this effect may also lead to dangerous situations, as particularly quick emergency movements are made difficult.
The invention is based on the task of improving the steering comfort.
With a hydraulic steering arrangement as mentioned in the introduction, this task is solved in that between the steering unit and the pump a supply line is arranged, in which a pressure reduction valve is arranged.
With this embodiment, hydraulic fluid in the required quantity and with the required pressure is available to the steering unit. As soon as the steering unit is activated, this pressure can reach the steering valve through the corresponding lines leaving the steering unit, to control or move said valve with the corresponding direction order.
When, however, the steering unit, which is substantially only provided for the activation of the steering valve, is supplied with the relatively high pump pressure, this pressure acts upon the inlet of the steering unit. It can easily be in range of, for example, 200 bars. In a normal steering system this is usually not a problem, as here the total flow from the pump connection of the steering unit goes to the load to be steered, that is, the wheels. In the system concerned, however, which works by means of a steering valve, this high pressure is not required, or is even disturbing. For the movement of the slide (or another steering element), the steering valve will do with a pressure of, for example, 5 bars. Accordingly, the corresponding pressure drop would have to be effected in the steering certain heat. Additionally, there is a risk that further to the noise and heat problems, also stability problems could occur, as for the disintegration of this high pressure difference only a very small activation of the steering unit can be permitted. Additionally, there is a risk that, in spite of all precautions, a too high pressure will reach the steering valve and activate it mechanically. For this reason, a pressure reduction valve is arranged in the supply line. This pressure reduction valve reduces the pump pressure to such a level that it no longer means any risk to the steering unit. Accordingly, the steering unit can still be working together with a steering valve. However, the noises and the heat development in the steering valve will be drastically reduced or even completely eliminated. In this connection, the supply line can be provided next to the available steering pressure line or supplement it. As now hydraulic fluid in sufficient quantity and with sufficient pressure is always available to the steering unit, problems with a xe2x80x9csubsequent supplyxe2x80x9d will not occur, so that the steering process can take place with the desired hydraulic support already from the beginning.
Preferably, the pressure reduction valve is made as a pressure limitation valve. This is particularly favourable in connection with varying pump pressures, as here the pressure is in fact limited to a maximum value. Here it should be mentioned that the term xe2x80x9cpump pressurexe2x80x9d does not only mean the pressure, which is available on the physical outlet of a pump. It can also be the pressure of a series-connected unit, for example the outlet of a priority valve, which again may comprise a pressure control valve.
Preferably, the steering valve is optionally operable by means of a steering handwheel or a hydraulic pressure produced by a control pressure generator. Here appears the particular advantage of the embodiment according to the invention. A steering valve, which is operable by means of a hydraulic pressure produced by a control pressure generator, is usually dimensioned for relatively small pressures. Higher pressures would make the electrical steering difficult and uneconomical. When now on the one hand it is provided that the steering unit receives a sufficient amount of hydraulic fluid with the corresponding pressure, on the other hand, however, this pressure is limited, it is still possible to combine the hydraulic steering via the steering handwheel and an electrohydraulic steering via the control pressure generator, without causing a risk of damaging or other negative influences.
Preferably, the pressure reduction valve is arranged in a position, in which it is acoustically and/or thermally decoupled from a place, in which the steering unit is arranged. Usually, the steering unit is arranged in the proximity of the steering handwheel. Having to be operated by the driver or the operating person, the steering handwheel must be arranged at the xe2x80x9cworking placexe2x80x9d, that is near the driver""s seat in a vehicle, which is equipped with the steering arrangement. In many modern machines, for example, agricultural working machines, this working place is surrounded by a cabin. This cabin also provides a certain noise insulation. When the pressure reduction was made in the cabin, for example, immediately next to the steering arrangement, the problem would occur that the noise, from which the cabin was supposed to protect, would be produced inside same cabin. With the embodiment shown, it can be ensured that the pressure reduction valve is arranged in a different place, for example, outside the driver""s cabin. This causes a substantial reduction of noises and heat in the cabin, also when the steering unit is placed here. Similar considerations also apply when no driver""s cabin is available. In this case, the pressure reduction valve can be placed at a distance from the driver""s seat, to keep the driver away from noises, which may be produced by the pressure reduction valve. Preferably, the supply line is connected with a return flow connection of the steering valve via a non-return valve opening in the direction of the supply line, which return flow connection is connected with the steering motor when the steering valve is activated. This connection is used with a so-called emergency steering, when the pump fails. In case of an emergency steering, it namely permits steering with the oil that returns from the steering motor, the steering unit acting as pump. This gives an emergency steering, which is faster than usual steerings, in which oil first had to be sucked in from the tank.